Heh, I just finished a book with a similar setting. Ages ago the surface was abandoned when their sun suddenly became poisonous (poisonous enough to cook within minutes, and give radiation poison to boot). A race did adapt, to the point they can last a few minutes under on sun (there were two, and the planet was a moon, so the day/night schedule was erratic). They also has suits that would let them survive the light a few hours. I thought it was entertaining the surface race was the dark skinned one, in a fantasy reversal. Anyways, the point is radiation might be a good route. It generally doesn't make sense under fantasy medicine, is uncurable, and can be fast acting.

As for maps, caves are 3d, but a regular topo map should suffice. This is because there is a ceiling. It may be useful to know that cave x goes over cave y at this point, but unless they connect it mostly doesn't matter. A cave system may start at one elevation, and end at another, but its the connection points that really matter. Also, depending on your world, they may be constantly changing, beyond your controlled kingdom areas, due to rock worms, are whatever other borrowing creatures you have making new routes and collapsing old one. Even just geological activity could cause enough unrest to make maps unusable.

Also, from way back, look at the Death Gate Cycle's Fire Sea, third book in the series. Similar world. And a good series.

@ Trae S, very cool! Yeah that's what I was thinking, a sort of radiation or somesuch that no one really understands how to fix or get rid of, and will stick around for millions of years, making the surface uninhabitable and very hostile. The idea here is that everyone long ago lost any hope or thought of returning to the surface...they are not in "exile" or anything, but rather have fully adapted to make the underworld their home. Very few people anymore entertain thoughts of returning "home" to the surface, because the underworld is now their home, the surface a distant memory of the ancient past.

OF course occasionally someone might make an expedition to the surface, for whatever reason...usually anthropological/archeological purposes, or maybe to try some harebrained idea of cleansing the atmosphere (that invariably fails and usually results in the deaths of everyone who made the trip).

The surface and underworld have switched roles from normal fantasy...the Underworld is the (somewhat) cultured, well-settled, safer regions, and the surface is the dreaded unknown and terrifyingly dangerous place where all manner of horrible things can kill you (mostly just the environment as not much else besides some Outsiders and undead can even exist there).

Of course, much of the Underworld still falls under the "unknown and terrifyingly dangerous place where things wait to kill you" category...but still.

And thanks for reminding me of Death Gate! I totally forgot about those books. I even have the Death Gate game for my DOS emulator!

I can't think of any settings that completely copy that theme, I mean there are the obvious similarities to some extent with other settings but that can be said about any of them.

I would read Brandon Sanderson's Mistborn triology if you haven't already. It's a world where the sun is almost permanently obscured by ash clouds spewed by half-active volcanoes. Someone already mentioned darksun for the whole apocalyptic aspect of it but that's not what I'm talking about particularly with regards to the book, I'm actually talking about how society developed because of their circumstances.

Because it was so hard for plants to grow it was considered extremely fashionable to have gardens, or because the ash gets everywhere having clean clothes is considered a sign of wealth and prosperity.

Think of how constantly being in the dark would change things, maybe they discovered long ago a type of mineral that functions well as fuel for lights and they use that as currency as well as or instead of gold and other precious metals. Perhaps because they need sunlight to grow, regular vegetables become an amazingly exotic delicacy and fungus/meat the norm.

I wouldn't worry about trying to decide on details about what exactly caused the surface to become uninhabitable. Think of the Mournland of Eberron. It's just an apocalyptic wasteland that was caused by a magical catastrophe (as far as anyone can figure), and no one knows anything more than that - including the GM/players.

Be able and willing to identify when it would be more trouble than its worth to fill things in, or more fun to leave them blank. It provides space for GMs to customize the setting to their own campaign needs and desires.

Speaking as a player, the first thing that appealed to me was adventuring across the surface, honestly. And I would hope to be able to in some way, even if not til I reach a high level.
Speaking as a DM, I have to admit that if you publish or otherwise release this material to the public, I would likely modify it (and I imagine most other DMs would as well) so that some of my campaigns may either take place up there, or more likely, just visit there at some point.
I realize that you want the focus to be centered on the Underground, but denying your players such a potentially rich place to adventure would rather disappointing.